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Friday, July 28, 2006

The Smallest Microphone

I've written about carbon nanotubes, calling them the next big thing, and about how we can use them to detect waves (like sunlight, or acoustic waves) of various types. In order to do that, you have to have a way to connect to the nanotubes. That's done by actually growing them on a junction instead of trying to place them there, which is very difficult. The image shows a 1mm X 6mm silicon wafer that has a forest of carbon nanotubes grown all over the surface in a specific pattern, which helps separate the nanotubes, keep them from shorting each other out (unless that is what you want) and even allows each nanotube to act like a small capacitive whisker that can move when perturbed by, say, a soundwave.

All the colors are caused by diffraction of light. Diffraction occurs on a CD. Extreme diffraction is what we get in this case because the rows of nanotubes are closely spaced - on the order of light wavelengths - and also very small at about 1000 nanometers tall.

That's small. 1000 nanometers is one micron. To put that in perspective, typical ribbon mics use an aluminum ribbon that is about 0.6 micron to about 2.5 micron thick (or thin, as we would see it).

Now since our silicon wafer can also be an amplifier and have other circuits etched or made in and on it, we think we can make a complete amplified sound sensor on a chip. The interesting thing is that the bandwidth, or frequency response, of carbon nanotubes on silicon, is thought to be length dependent. That means that various heights of nanotubes can pick up certain parts of the audio spectrum (and RF spectrum too). Adjusting the heights of the tubes can allow us to tailor the frequency pickup. That's the idea. Away we go, off to get it done.